Materials implanted in vivo essentially have direct contact with the human body through the interface between the implant surface and bones, tissues and extracellular body fluids. The surface of the implant therefore plays a very important role related to surface chemistry, topography and micro/nano structure, and tribological properties. Major issues related to surface modification processes include corrosion and wear resistance of the implant and biocompatibility and bioactivity. Chloride ion concentration in body fluid is 113 mEql−1 and in interstitial fluid is 117 mEql−1, which may corrode metallic materials. Body fluids contain amino acids and proteins that tend to accelerate corrosion. Toxicity and allergy occur if metallic materials are corroded by fluid, if metallic ions are released into the fluid for a long time, or if ions combine with biomolecules such as proteins and enzymes. Loosening of implant could occur due to the wear of the implant. All of these factors lead to premature implant failures, debilitating pain, and surgical revisions.
Both corrosion and wear are related to the surface of implants. Extensive studies have been reported on surface modifications to understand and enhance the performance of implants. One approach is to modify the surface topography by creating a rough or porous surface on the implant to increase the surface area available for bone/implant apposition, which improves the fixation of the implant in the bone. A natural consequence of increasing the surface area is an increase in metal ion release, due to an increased surface contact with corrosive media. A further complication is the increase in wear debris due to increased surface friction, which also results in increased ion release rates and loosening of the implants. Another approach to surface modification is to coat the implant surface with hard materials focusing on increasing the wear resistance. Titanium nitride was extensively reported for implant surface modification using chemical vapor deposition (CVD) and physical vapor deposition (PVD). Although these methods provide the implant articulating surfaces with excellent wear resistance, the deposited layers often suffer from lack of adherence and are not associated with bone/implant apposition. Low energy nitrogen ion bombardment-plasma nitriding is one of the most up-to-date methods for improving the wear and corrosion behavior of metallic alloy. In plasma nitriding, a Ti-based substrate is directly involved in the reaction of coating formation, which results in an excellent adhesion of the coating to the substrate. However, the inherent high cost of plasma nitriding equipment and its operation reduces its cost-effectiveness.
Other surface coatings have been tried to improve the bone/implant interface bonding. These include hydroxyapatite (HA) coatings produced by plasma spray or ion implantation. Hydroxyapatite, (Ca10(PO4)6(OH)2), is characterized by a hexagonal structure (a=9.423 Å, c=6.875 Å, Space Group: P63m) with a density of 3.16 g/cm3. It is one of the three main components of the human body (HA, water and collagen) and is able to integrate bone structure and support bone ingrowth. For this reason, coatings of hydroxyapatite are often applied to metallic implants to alter the surface properties. In this manner the body sees the hydroxyapatite-type material as a compatible material. Without the coating, the body would see a foreign body and either isolate it from surrounding tissues or induce a tissue reaction.
However, HA coatings formed by plasma spray, the most popular commercially available technique for HA coating on implants, generated some long term concerns. A study has revealed that even though uncemented HA-coated hip prostheses had better survivorship than cemented, the HA cups with follow-up longer than 6 years revealed an increased surgical revision rate (replacement of the primary implant). Also in a study about polyethylene wear, osteolysis and acetabular loosening with HA-coated prostheses, there were no stem revisions but 24% of the acetabular components required revision. HA debris might accelerate the wear of the high density polyethylene material (HDPE) of the acetabular component. Another study of HA coating on a G.B. acetabular cup found a high rate of debonding and failure. Yet another study reporting on the evaluation of 6 revisions of HA-coated acetabular cups showed HA granules embedded in the HDPE, which may produce severe clinical problems.
There are a variety of known techniques to produce HA coatings on substrate surfaces. Plasma spray advantageously produces high density coatings, but disadvantageously is a line-of-sight process and oxidation of powder when conducted in air leads to poor adhesion and low purity coating. High velocity oxyfuel advantageously provides good coating bond strength, but disadvantageously is a line-of-sight process and produces low purity coatings. Ion bean assisted deposition advantageously uses low deposition temperature, provides high adhesion and provides good control of stress level, microstructure and composition, but disadvantageously is a line-of-sight process and is higher in cost. Pulsed laser deposition advantageously provides high purity coatings, but disadvantageously is a line-of-sight process, requires high capital investment and maintenance costs and provides a low deposition rate. Chemical vapor deposition advantageously is not line-of-sight dependent, readily provides coatings at near theoretical density and permits control of preferred grain orientation and grain size, but disadvantageously is a high temperature process in most cases leading to low purity HA coatings. Electrodeposition is advantageously low cost and simple and provides uniform coatings of high purity and low porosity, but disadvantageously is a line-of-sight process to some extent and is a two-step process that must be followed by hydrothermal treatment to obtain HA coatings. Electrophoresis deposition advantageously is not a line-of-sight process, is low cost and simple, provides high deposition rate and produces a wide range of coating thicknesses (from <1 um to >500 um), but disadvantageously is a two-step process requiring densification by sintering which may reduce the purity of the HA coating. Sol-gel deposition advantageously permits coating of complex shapes with coatings having increased homogeneity and fine-grained structures, but disadvantageously requires firing leading to reduced purity of the HA coating. Bio-mimetic deposition advantageously is a low temperature process applicable to any heat sensitive surface including polymers, permits formation of bone-like apatite crystals with high bioactivity and permits incorporation of bone growth stimulating factors and antibiotics, but disadvantageously is a very slow process requiring precise control of process parameters in which obtaining uniform coating is a practical challenge.
Finally, chemical deposition is a process mainly used to prepare HA powders but not to coat HA on a substrate. Few studies of chemical deposition of bioceramic materials are available and process kinetics are poorly understood. In theory, chemical deposition may be able to provide uniform coatings of unlimited thickness on complex shapes, be used to deposit HA on polymer surfaces, and produce a porous top layer to encourage bone ingrowth. No suitable chemical processes are commercially available for coating.
Of the processes described above, most are line-of-sight dependent and/or involve high temperature (over 15,000° C. for plasma spray). It is a challenge for any process that is line-of-sight in nature to produce uniform coating, particularly on sloped and curved surfaces. As for processes that rely on high temperature, they cause decomposition of HA which leads to the formation of impurities such as tetracalcium phosphate (Ca4P2O9), amorphous calcium phosphate, α-tricalcium phosphate (Ca3(PO4)2), and β-tricalcium phosphate (Ca3(PO4)2). These impurities are unstable in the body fluids and cause serious concerns for localized corrosion. The selective dissolution of these impurities may result in an accelerated wear caused by the roughening/scoring of the articulating surface, and this debris will, in turn, make the wear a more severe issue.
It is apparent from the processes described above that biomimetic and chemical processes are neither line-of-sight dependant nor involve high temperature operation. Biomimetic coating is an approach that consists of immersion of metal implants in simulated body fluids (SBF) at a physiologic temperature and pH. HA coating, the major component of bone, grows in a way similar to the natural bone growth in our body. This process produces HA coating with desirable properties such as high purity and bioactivity. Another uniqueness of this process is its capability to incorporate antibiotics (e.g. tobramycin), proteins and bone growth stimulators (e.g. osteogenics). However, although SBF mimics the inorganic composition, pH, and temperature of human blood plasma, achieving a reasonable coating thickness for practical applications takes a long time. Long immersion time (7-14 days) with daily refreshment of SBF's is required. The difficulty results from the metastability of SBF and the process requires replenishment and a constant pH to maintain supersaturation for apatite crystal growth. As a result of the low solubility product of HA and the limited concentration range for the metastable phase, this operation is extremely difficult and might lead to local precipitation or uneven coatings. Such an intricate and long process can hardly be tolerated in the prostheses coating industry.
Chemical coating processes produce HA coatings at low temperature and are line-of-sight independent. Theoretically, chemical processes can produce uniform coatings of unlimited thickness on complex shapes. The deposition rate of chemical HA coating is significantly higher than the biomimetic process due to significantly higher and controllable process parameters. Unfortunately, little is known about its chemical reaction kinetics and the process is used mainly for producing HA powder.
There remains a need in the art for a chemical process for coating a bioceramic material, e.g. hydroxyapatite (HA), on a surface of a substrate.